The present invention relates generally to a method for thermally compensating pressure gauge determinations, and to means for providing an accurate pressure determination. More particularly, the invention relates to a method for thermally compensating quartz pressure gauge determinations through the use of a dynamic model of the thermal response of the resonator of the pressure gauge. The method provides for quick and accurate thermal compensation which is especially useful in environments such as encountered in the formation testing, oil well and borehole arts where substantial thermal gradients and/or thermal shocks, some of which are due to pressure shocks and resultant adiabatic heating or cooling, may be present.
In the pressure gauge arts, pressure gauges employing precision piezoelectric quartz resonators are known for providing highly accurate pressure determinations. For example, as disclosed in U.S. Pat. No. 3,561,832 to H. E. Kerrer et al., which is assigned to the Hewlett-Packard Company, a quartz pressure gauge may be fashioned from a single piece of crystalline quartz with the gauge comprising a hollowed cylinder and a circular resonator with electronics thereupon. The cylinder may be oriented with respect to the crystalline axis for selected characteristics. By applying a predetermined voltage to the resonator, the resonator may be caused to resonate at a resonance frequency. The application of pressure around the cylinder causes the resonator to change its oscillation frequency. By measuring the change in the oscillation frequency, the applied pressure may be accurately determined.
While the oscillation frequency of the resonator of the quartz pressure gauge is a function of the applied pressure, it is known that the frequency is also affected by temperature. Thus, in order to provide an exact measure of the applied pressure, the reading must be compensated for temperature changes. One proposed method for eliminating the temperature effect on the pressure measuring gauge was to provide an isolated second (reference) gauge which was to be kept at a constant pressure but which was to be subjected to the identical temperature fluctuations of the first gauge. Under equilibrium, when both gauges had attained thermal equilibrium at the same temperature, the difference in frequencies output by the gauges could then be directly correlated to the change in pressure seen by the first gauge.
Other devices and techniques for compensating for the temperature dependence of pressure gauges are known. Typically, the devices provide compensation circuits which are sensitive to a measured temperature and cause either the input current and voltage or the output reading to change as a function of the measured temperature. An example of such devices is U.S. Pat. No. 4,414,853 to J. Bryzek, assigned to the Foxboro Company, where the voltage supply of a pressure transducer is regulated by a circuit which responds to both the pressure determination and a feedback circuit operating in a non-linear manner for generating a control signal in response to a temperature signal. Through such a scheme, the current supplied to the pressure sensor is changed so that compensation is made for errors in pressure measurement caused by changes in temperature. Another example of temperature compensation for pressure gauges is found in U.S. Pat. No. 4,366,714 to N. Adorni, assigned to CISE S.p.A., where a compensation technique is disclosed for particular application to drill wells of great depth. The patent discloses an apparatus for pressure/temperature measuring, where a separate temperature reading of the pressure sensor temperature is used to correct the pressure sensor reading through the utilization of calibration procedures. Those skilled in the oil well and borehole arts will appreciate that accurate pressure measurements and a log of the pressure over time and/or along the depth of the borehole are advantageous. Pressure measurements are used in a wide variety of geophysical logging tools, such as, for example, the formation-sampling apparatus disclosed in U.S. Pat. No. 3,530,933 to F. Whitten, and assigned to the same assignee of this invention.
While many different techniques and devices purportedly compensate pressure determination for thermal effects, all of those techniques known to the inventor have assumed that the temperature over the entire resonator of the pressure gauge is constant (static) during measurement. Because this assumption is only true where the gauge is permitted to reach thermal equilibrium, complete compensation for thermal effects is impossible in the prior art if the environmental temperature is changing, such as in the well logging and formation testing arts where pressure pulses causing adiabatic heating are being measured. In situations where the temperature at a given point is constant but measurement is desired over a distance such that the gauge is moved through areas of differing temperatures, complete compensation is obtained by the prior art only by permitting the gauge to remain at a fixed location for a long period of time. Such time expenditure, however, is disadvantageous in many applications, and especially in the borehole and production logging arts, where thermal gradients may be large or thermal shocks prevalent.